Oxidation resistant coatings for molybdenum

ABSTRACT

A molybdenum article is disclosed in which the substrate thereof has adherently bonded thereto a thermally self-healing plasma-sprayed coating consisting essentially of a composite of molybdenum and a refractory oxide material capable of reacting with molybdenum oxide under oxidizing conditions to form a substantially thermally stable refractory compound of molybdenum. The plasma-sprayed coating is formed of a plurality of interbonded plasma-sprayed layers of a composite of molybdenum/refractory oxide material produced from a particulate mixture thereof. The coating comprises a first layer of molybdenum plasma-sprayed bonded to the substrate of said molybdenum element and a second layer of plasma-sprayed mixture of particulate molybdenum/refractory oxide consisting essentially of predominantly molybdenum bonded to the first layer. The succeeding layers of the molybdenum/refractory oxide mixture provide from layer to layer a descending concentration gradient of molybdenum and a corresponding ascending concentration gradient of refractory oxide material. The concentration gradient continues to an N-1 layer in which the refractory oxide concentration of the mixture predominates and terminates at the Nth layer, the Nth layer consisting essentially of the refractory oxide material plasma-sprayed bonded to the N-1 layer.

This invention relates to protectively coated molybdenum elements orarticles for use at elevated temperatures in oxidizing and/or corrosiveenvironments. Examples of such articles include molybdenum electrodesfor use in molten glass environments, nozzles, nozzle vanes and otherstructural components employed in jet engines, among other molybdenumcomponents.

STATE OF THE ART

As a refractory metal, molybdenum is very attractive for use at elevatedtemperatures. It has a high melting point of about 2610° C. and adensity of about 10.28 grs/c.c.

Molybdenum exhibits high resistance to creep and deformation atrelatively high temperatures. In addition, molybdenum maintains arelatively high strength at elevated temperatures.

A drawback in the use of molybdenum at elevated temperatures inoxidizing environments is its increasingly susceptibility to oxidationat temperatures above 1100° F. (595° C.) due to the formation ofmolybdenum trioxide which becomes highly volatile at temperatures above1650° F. (900° C.). Because of this, when the metal is exposed to hightemperatures, a rapid oxidation takes place and the metal issubstantially destroyed by continual oxygen attack and volatization ofmolybdenum trioxide. The attack progresses from the surface of themolybdenum substrate to the core of the exposed article.

With reference to the disclosure of U.S. Pat. No. 2,683,305, attemptshave been made to protect molybdenum surfaces by conventional means,such as by enveloping the metal in a stable ceramic, for example,alumina or silimanite, in the form of shells or tubes. Another methodattempted has been to spray the ceramic material in the form of a slurryand subsequently baking the coating to the substrate. Metal coatingshave been tried, either by electrolysis or vapor deposition, among othermethods. Despite these attempts, the coatings or casings did not achievelasting results. The coatings tended to be porous and to spall duringthermal cycling, thereby exposing the molybdenum substrate tocatastrophic oxidation.

Molybdenum, has a high coefficient of diffusion at elevatedtemperatures. It can diffuse into an alloy coating at a rapid rate withincrease in temperature. Stable compounds, such as the refractory oxidesAl₂ O₃, SiO₂ or MgO when brought into contact with molybdenum oxide athigh temperatures react to form molybdate compounds.

It would be desirable to provide a protective, strongly adhering coatingwhich not only resists the inward diffusion of oxygen from theenvironment, but which in turn resists the outward diffusion ofmolybdenum trioxide into or through the coating. The coating should beone that resists spalling during thermal cycling and which in use isself-healing.

OBJECTS OF THE INVENTION

One object of the invention is to provide a coated molybdenum articlewhich will resist oxidation and corrosion at elevated temperatures.

Another object is to provide a coated molybdenum article in which thecoating is formed of a plurality of layers and which is self-healing atelevated temperatures under oxidizing conditions.

These and other objects will more clearly appear when taken inconjunction with the following disclosure, the appended claims and theaccompanying drawing.

THE DRAWING

FIG. 1 is a cross-section of a coated molybdenum substrate which hasbeen exaggerated for clarity to show schematically the layers making upthe coating; and

FIG. 2 is similar to FIG. 1 except that it is a specific embodiment inwhich the coating is made up of eleven layers produced by plasmaspraying to provide a composition gradient based on a mixture ofparticulate molybdenum and refractory oxide, the molybdenum contentdecreasing from layer to layer while the refractory oxide contentincreases.

SUMMARY OF THE INVENTION

Stating it broadly, the invention is directed to an article ofmanufacture in the form of a molybdenum element in which the substratethereof has adherently bonded thereto a thermally self-healingplasma-sprayed coating consisting essentially of a composite ofmolybdenum and a refractory oxide material capable of reacting withmolybdenum oxide under oxidizing conditions to form a substantiallythermally stable refractory compound of molybdenum. The plasma-sprayedcoating is formed of a plurality of interbonded plasma-sprayed layers ofa composite of molybdenum/refractory oxide material produced from aparticulate mixture thereof, the plurality of bonded layers ranging froma first layer adjacent to molybdenum substrate to an Nth or last layer.The first layer is formed of molybdenum which is plasma-sprayed bondedto the substrate of molybdenum element.

The second layer comprises a plasma-sprayed mixture of particulatemolybdenum/refractory oxide consisting essentially of predominantlymolybdenum bonded to the first layer, with the succeeding layers of themolybdenum/refractory oxide mixture providing from layer to layer adescending concentration gradient for molybdenum and a correspondingascending concentration gradient for the refractory oxide material, theconcentration gradient continuing to an N-1 layer in which therefractory oxide concentration of the mixture predominates andterminates at the Nth layer. The Nth layer consists essentially ofrefractory oxide material plasma-sprayed bonded to the N-1 layer.

The refractory oxide is preferably selected from the group consisting ofzirconium oxide, hafnium oxide and aluminum oxide, aluminum oxide beingpreferred. Any refractory oxide can be used so long as it is capable ofreacting with molybdenum powder under oxidizing conditions to producestable, complex, molybdenum oxide compounds having melting points inexcess of about 950° C.

The layers between the first and last layers making up the coating areproduced by plasma spraying a mixture of molybdenum and refractory oxidepowders, with the molybdenum content of the mixture descending inconcentration from layer to layer as the refractory oxide contentascends in concentration, thereby providing a concentration gradienthaving improved resistance to spalling as well as improved resistance tooxidation and corrosion. The particle size of the mixtures employedgenerally ranges from about 20 microns to about 200 microns.

The preferred coating is one in which the descending concentration formolybdenum from layer to layer proceeds step-wise at approximately 10%increments by weight, while the corresponding ascending concentrationfor the refractory oxide from layer to layer proceeds step-wiseapproximately 10% increments by weight, with the last layer beingsubstantially all refractory oxide.

DETAILS OF THE INVENTION

The layers forming the total coating are applied using plasma spraying.Plasma spraying is a particular method of spraying whereby a gas, e.g.,argon, nitrogen, and hydrogen, is caused by virtue of its passagethrough an electric arc to be put into highly excited state. The statecorresponds to a higher energy state than the gaseous state and providestemperatures substantially in excess of 5000° C., for example in excessof 10,000° C. and up to about 15,000° C. and higher.

A powder-type plasma spray gun is used in producing the coatings. Anexample of such a gun is the 3MB plasma gun which is marketed by Metco,Inc. of Westbury, Long Island. This gun uses a GE nozzle through whichthe powder to be sprayed is fed using argon as a primary gas at elevatedpressure. The powder passes through the plasma arc and is melted priorto reaching the substrate.

As pointed out in U.S. Pat. No. 3,958,097, during plasma spraying, anarc is established between two oppositely polarized electrodes employinga current generally in the range of 155 to 1000 amps. The powder to besprayed is forced through a constricting orifice and accelerated atsonic velocities with intense heat.

Because of the ease with which molybdenum powder oxidizes, an annulargas stream is used as a shield, i.e., argon, surrounding the heatedpowder exiting from the nozzle, thereby assuring protection for themolybdenum powder mixed with the refractory. An advantage of usingplasma spraying is that a uniform deposit is obtained accompanied bystrong adherent bonding to the substrate, as well as bonding between thesubsequently deposited layers.

A schematic cross-section is shown in FIG. 1 broken away showing atypical coating starting with a first layer of plasma-sprayed molybdenumfollowed by succeeding layers of molybdenum and refractory oxidemixture.

FIG. 1 shows a molybdenum substrate 10 upon which a molybdenum layer 11has been plasma-sprayed followed by a next layer 12 of a plasma-sprayedmolybdenum/refractory oxide mixture in which molybdenum predominates,and so on to the "N-1" layer 13 in which the refractory oxidepredominates, the last plasma sprayed layer "N" (14) being substantiallyall refractory oxide.

The plurality of layers provide a composition gradient which aids ininhibiting spalling by averaging out the expansion coefficientdifferences from layer to layer and thus avoid sudden changes in thermalexpansion across the coating during thermal cycling. Each layer has adispersion of molybdenum particles which serve to capture oxygendiffusing inward from the environment at elevated temperatures andthereby resist the further penetration of oxygen to the molybdenumsubstrate being protected. As the molybdenum particles react with theoxygen, the oxidized molybdenum reacts with adjacent particles ofrefractory oxide to form complex stable compounds of molybdenum oxide.

Thus, the coating is self-healing and provides protection to themolybdenum substrate. This can be important in instances where microfissures, if any, form during thermal cycling.

FIG. 2 is illustrative of a specific embodiment of the invention whereinthe descending concentration gradient for molybdenum proceeds atstep-wise increments of approximately 10% by weight from layer to layer,while the corresponding ascending concentration gradient for therefractory oxide proceeds at step-wise increments of approximately 10%by weight.

As illustrative of the invention, the following examples are given:

EXAMPLE 1

A protective coating is produced on a molybdenum substrate by employingalumina as the refractory material. A plurality of powder charges areemployed to provide the desired concentration gradients in the ultimatecoating. In this example, the powder charges vary from each other atcomposition increments of approximately 20% by weight as follows:

First Layer--Molybdenum powder of average particle size of about 20 to200 microns.

Second Layer--80% Mo/20% Al₂ O₃ of average particle size of about 20 to200 microns.

Third Layer--60% MO/40% Al₂ O₃ of average particle size of about 20 to200 microns.

Fourth Layer--40% Mo/60% Al₂ O₃ of average particle size of about 20 to200 microns.

Fifth Layer--20% MO/80% Al₂ O₃ of average particle size of about 20 to200 microns.

Sixth Layer--100% Al₂ O₃ of average particle size of about 20 to 200microns.

The molybdenum substrate is cleaned prior to plasma spraying in theconventional manner.

Thereafter, each of the layers is deposited by plasma spraying in theknown manner to provide a coating formed of a plurality of layers inwhich the concentration gradient from the first layer to the last layerproceeds at descending increments for molybdenum of approximately 20% byweight and corresponding ascending increments for alumina ofapproximately 20% by weight.

EXAMPLE 2

The method of Example 2 is repeated except that the descending andascending increments of concentration of molybdenum and alumina proceedat approximately 10% by weight.

EXAMPLE 3

A protective coating is produced on a molybdenum substrate by employingzirconia as the refractory material using the process described inExample 1. The powder charges employed for each layer vary from eachother at composition increments of approximately 10% by weight as inExample 2, the first layer deposited being substantially all molybdenum,the last layer deposited being substantially all zirconia.

Following completion of the coating process, it may be desirable tostabilize the coating before use by heating the coated molybdenumarticle to an elevated temperature in excess of about 800° C. andranging up to about 1100° C. in an oxidizing atmosphere in order toremove any porosity resulting from the plasma spray process. By allowingthe oxygen from the atmosphere to penetrate the coating and react withthe dispersed molybdenum particles, the coating self-heals and theporosity of the coating is substantially eliminated. The heating may beconducted at temperature for at least about 2 hrs and range up to about10 hrs. When the coated component is to be used at elevated temperaturesin excess of 800° C. in oxidizing atmpopheres, the coating may bestabilized during the initial stages of use.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and the appended claims.

What is claimed is:
 1. As an article of manufacture, a molybdenumelement comprising a molybdenum substrate in which said substrate hasadherently bonded thereto a thermally self-healing plasma-sprayedcoating consisting essentially of a composite of molybdenum and arefractory oxide material capable of reacting with molybdenum oxideunder oxidizing conditions to form a substantially thermally stablerefractory compound of molybdenum,said plasma-sprayed coating beingformed of a plurality of interbonded plasma-sprayed layers of acomposite of molybdenum/refractory oxide material produced from aparticulate mixture thereof, said plurality of bonded layers rangingfrom a first layer adjacent to said substrate to an Nth or last layerwhich include:(a) a first layer of molybdenum plasma-sprayed bonded tothe said molybdenum substrate of said molybdenum element, (b) a secondlayer of plasma-sprayed mixture of particulate molybdenum/refractoryoxide consisting essentially of predominantly molydbenum bonded to saidfirst layer, (c) with succeeding layers of said molybdenum/refractoryoxide mixture providing from layer to layer a descending concentrationgradient of molydbenum and a corresponding ascending concentrationgradient of refractory oxide material,said concentration gradientcontinuing to an N-1 layer in which the refractory oxide concentrationof the mixture predominates and terminates at the Nth layer, (d) saidNth layer consisting essentially of said refractory oxide materialplasma-sprayed bonded to said N-1 layer.
 2. The article of manufactureof claim 1, wherein said refractory oxide material is selected from thegroup consisting of zirconium oxide, hafnium oxide and aluminum oxide.3. The article of manufacture of claim 2, wherein said refractory oxideis aluminum oxide.
 4. As an article of manufacture, a molybdenum elementcomprising a molybdenum substrate in which said substrate has adherentlybonded thereto a thermally self-healing plasma-sprayed coatingconsisting essentially of a composite of molybdenum and a refractoryoxide material selected from the group consisting of zirconium oxide,hafnium and aluminum oxide, which refractory oxide material is capablereacting with molybdenum oxide under oxidizing conditions to form asubstantially thermally stable refractory compound of molybdenum,saidplasma-sprayed coating being formed of a plurality of interbondedplasma-sprayed layers of a composite of molybdenum/and said refractoryoxide material produced from a particulate mixture thereof, saidplurality of bonded layers ranging from a first layer adjacent to saidsubstrate to an Nth of last layer which include:(a) a first layer ofmolybdenum plasma-sprayed bonded to the said molybdenum substrate ofsaid molybdenum element, (b) a second layer of a plasma-sprayed mixtureof said particulate molybdenum/refractory oxide consisting essentiallyof predominantly molybdenum bonded to said first layer, (c) withsucceeding layers of said molybdenum/refractory oxide mixture providingfrom layer to layer a descending concentration gradient of molybdenumand a corresponding ascending concentration gradient of refractory oxidematerial,said concentration gradient continuing to an N-1 layer in whichthe refractory oxide concentration of the mixture predominates andterminates at the Nth layer, (d) said Nth layer consisting essentiallyof said refractory oxide material plasma-sprayed bonded to said N-1layer.
 5. The article of manufacture of claim 4, wherein said succeedinglayers of said plasma-sprayed molybdenum/refractory oxide mixturestarting from said first layer of plasma-sprayed molybdenum provides alayer to layer descending concentration gradient of molybdenum atincrements of approximately 10% by weight and layer to layer ascendingconcentration gradient of the refractory oxide at incrementsapproximately 10% by weight.
 6. The article of manufacture of claim 5,wherein said refractory oxide is aluminum oxide.
 7. As an article ofmanufacture, a molybdenum element comprising a molybdenum substrate inwhich said substrate thereof has adherently bonded thereto a thermallyself-healing plasma-sprayed coating consisting essentially of acomposite of molybdenum and aluminum oxide which reacts with molybdenumoxide under oxidizing conditions to form a substantially thermallystable refractory compound of molybdenum,said plasma-sprayed coatingbeing formed of a plurality of interbonded plasma-sprayed layers of acomposite of molybdenum/aluminum oxide material produced from aparticulate mixture thereof, said plurality of bonded layers rangingfrom a first layer adjacent to said substrate to an Nth or last layerwhich include:(a) a first layer of molybdenum plasma-sprayed bonded tothe said molybdenum substrate of said molybdenum element, (b) a secondlayer of plasma-sprayed mixture of particulate molybdenum/aluminum oxideconsisting essentially of predominantly molybdenum bonded to said firstlayer, (c) with succeeding layers of said molybdenum/aluminum oxidemixture providing from layer to layer a descending concentrationgradient of molybdenum at approximately 10% increments and acorresponding ascending concentration gradient of refractory oxidematerial at approximately 10% increments,said concentration gradientcontinuing to an N-1 layer in which the aluminum oxide concentration ofthe mixture is approximately 90% and terminates at the Nth layer, (d)said Nth layer containing approximately 100% of said aluminum oxidematerial plasma-sprayed bonded to said N-1 layer.